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BQ40Z80: RSOC value gradually increases

Yukinari Sato
Prodigy 45 points
Part Number: BQ40Z80

Hi,

When repeatedly charging and discharging, the value of RSOC is observed to gradually increase abnormally, and when the Scale Factor setting in Current Scaling is changed, the RSOC value shows a stable behavior.

Please refer to the attached file for more detail.

The specifications of the battery pack include
- Maximum charge and discharge current 180A
- Current detection resistance 0.1mΩ

I believe there may be a problem with the Scale Factor setting value. Please advise the cause of this and the correct solution. Please let me know the appropriate Scale Factor setting value.

Best regards,

Sato

3438.RSOC Issue (2).xlsx

  • +1
    TI__Guru** 105760 points

    Hello Sato,

    You should not use the scale factor with the BQ40Z80, there is an issue associated with the scale factor implementation in firmware. You should manually scale the current by applying 2A and calibrating the gauge and inputting 1A for calibration (for example). You will also need to adjust any capacity values along with this method.

    Also your sense resistor is too low for the gauge to accurately coulomb count unless your discharges are always very large (greater than 40mA). The minimum we support is 1mOhm sense resistor.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 45 points

    Hello Wyatt,

    Battery packs are in the mass production stage, and it is difficult to change the sense resistor.
    It is inevitable that a scale factor should not be used, but the RSOC accuracy is not that necessary, so please advise how we can manage to handle this with the current configuration.

    The setting [*1] that we have confirmed for RSOC in our experiments shows normal values, is there a problem with that setting? Is there any good way to do this?

    Any help would be appreciated.

    Best Regards,

    Sato

  • 0 in reply to Yukinari Sato
    TI__Guru** 105760 points

    Hello Sato,

    It is okay if it cannot be changed, but just be aware it can lead to lower resolution and noise in the current measurement.

    If it is directly related to the scale factor being used in the gauge, you will need to do the process I outlined previously. You must force a certain current (maybe for your case 10A since your sense resistor is so small) and tell the gauge that it is actually 5A, for example, to scale the value by 1/2. You will also need to scale all the capacity values configured for DF when you do this.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 45 points

    Hello Wyatt,

    Thank you for your quick response.


    1) Disable Scale Factor by setting it from 1 to 0 (magnification remains the same)
    2) Set the sense resistor to 10 times to make the current be recognized as 1/10.
    3) Set the battery capacity to 1/10.

    Is it OK to say that these settings will not be affected by firmware problems and stable RSOC will be obtained?

    If this approach is correct, this case is closed.

    Best regards,

    Sato

  • +1 in reply to Yukinari Sato
    TI__Guru** 105760 points

    Hello Sato,

    Yes that's correct, just make sure you adjust all values that are in unit of mAh or mA to the appropriate scale factor you used.

    Sincerely,

    Wyatt Keller

  • +1 in reply to Wyatt Keller
    Prodigy 45 points

    Hello, Wyatt,

    Understood. We will evaluate with this setting.
    Thank you for your response.

    Best Regards,

    Sato

  • 0 in reply to Wyatt Keller
    Prodigy 45 points

    Hello Wyatt,

    Let me confirm one more point.

    Is it correct to understand that there is no difference in basic performance and characteristics such as electrical characteristics, protection functions, and capacitance accuracy , between "the Scale Factor is set to 0" and "the Scale Factor is set to 10 " ?   (current and capacity value are scaled appropriately)

    Since we have not been able to evaluate anything other than RSOC when the scale factor is disabled, we are concerned about whether there is any harm in disabling the scale factor.

    Best regards,

    Sato

  • +1 in reply to Yukinari Sato
    TI__Guru** 105760 points

    Hello Sato,

    That's correct, everything should be the same you just need to do the scale factor manually be calibrating the gauge as I mentioned before instead of using the scaling factor that would do it internally.

    Sincerely,

    Wyatt Keller